Multichannel signaling system



v March 10, 1942. P. F. BYRNE ET AL 2,276,154

MULTICHANNEL SIGNALING `SYSTEM y Filed Aug. 9, 1938 5 sheets-sheet 1 March lO, 1942.* P. F. BYRNE ETAL MULTIGHANNELIGNALING sYsTM s shee'ts-sheet 2 Filed Aug. 9, l1938 QN. Q N

RQ N ung www March 10, 1942. Pl F. BYRN ETAL 2,276,154 f MULTICHANNEL SIGNALING`SYSTEM Filed Aug. 9, 1938 s sheets-sheet s ATTORNEY f Patented Mar. 10, 1942 UNETED STATES MULTICHANNEL SIGNALING SYSTEM Application August 9, 1938, Serial No. 223,848 I (Cl. Z50-9) 10 Claims.

The present invention relates to multi-channel systems wherein a radio channel constitutes at least part of the common path, although the invention may also be practised in a system wherein the common path is constituted by physical conductors.

It is an object of the invention to provide a system wherein a large number of series of telegraph signals or messages each having a high keying speed or dotting frequency may be transmitted over va common path on a number of relatively closely spaced carriers so that the complete width of the frequency band required for transmitting all the signals is narrower than has hitherto been possible in previously known systems. In particular it is an object to transmit eight to fourteen series of high speed telegraph signals or messages having dotting frequencies -between 200 and 300 words per minute over a number of carriers of audio frequency lying between 1,000 and 6,000 or 1,000 and 10,000 cycles, depending upon the number of channels. It is a further object to transmit such a number of high speed signals over such a narrow spectrum by restricting at both the transmitter and the receiver the higher harmonics of the dotting frequencies of telegraph signals, while at the same time arranging the system so that the delivered signals from the receiver are substantially undistorted in proportion or shape, and have a substantially square-topped form.

It is a further object of the invention to provide. such a multi-channel telegraph system in which interference due to cross-talk between adjacent channels or due to extraneous signal noise picked up in the common signaling path may be substantially eliminated.

It is a further object to provide a system in which the effects of fading in the common transmission path are compensated so that in spite of such fading the signals delivered by the receiver are of substantially constant amplitude. It is an especial object to provide such a system wherein the attainment of substantially constant amplitude does not result in a change in the weight of the signals, i. e. the ratio of the dot or dash intervals to the space intervals therebetween.

In accordance with one feature of the present invention a comparatively large number of high speed telegraph signals are transmitted over lli audio frequency carriers comparatively closely spaced, this close spacing being made possible by providing filters at both the transmitter and receiver Whose transmission characteristics are such as to substantially eliminate the third and higher harmonics of the dotting frequency of the telegraphic signals. In order to compensate the loss of definition in the signals which results from such elimination of the third and higher harmonics by the transmitter and receiver filters, limiting means are provided in accordance with a further feature of the invention to reshape the distorted waves at the receiver so as to give these waves an essentially fiat-topped form; and in accordance with still a further feature of the invention, these limiting means are automatically adjusted so that in spite of fading of the incoming signals the reshaping operation of the limiter does not cause a variation in the weigh or marking/spacing ratio of Ithe signals delivered from the receiver. This automatic adjustment of the limiting means is performed by Varying the critical level at which the limiter cuts off in dependence upon the mean amplitude of the corresponding incoming signals.

In accordance with still another feature of the present invention, the variations of the critical levels of the various limiters are further controlled in a'mutual fashion so that the level to which each limiter is adjusted also depends upon the mean amplitude ofthe received signals in other channels. This feature prevents the limiter of an idle or non-working channel from adjusting itself to such ahigh sensitivity that crosstalk will result into this idle channel from the other active channels of the system.

In accordance with another feature of the invention, interference of the so-called static type which consists predominantly of sharp transient surges having a frequency distribution which extends over a wide spectrum and ordinarily having an amplitude many times greater than the amplitude of the transmitted signals, is minimized by the use vof two filters with a limiter between them in the receiving part of the system, the lter which follows the limiter being substantially sharper in its characteristic than the lter which precedes the limiter.

The exact manner of practising my invention and the novelty and advantages thereof may best be understood from the following detailed description taken in connection with the drawings, in which- Fig. 1 represents the transmitter portion of a multi-channel telegraph system in accordance with my invention;

Fig. 2 represents the receiver portion of such a multi-channel telegraph system, and

Fig. 3 illustrates the circuit details of one of the audio amplifier and limiter devices shown in Fig. 2.

Referring more particularly to Fig. 1, la is a tape actuated keying device driven by a motor at such a speed as to transmit keying signals at the rate of 200 to 300 words per minute corre-k sponding to a dotting frequency of 80 to 120 cycles per second. The dotting frequency is understood to be the highest frequency at which separate pulses of signal are transmitted at a given keying speed. Thus the duration of the shortest possible dot plus the shortest possible space following the dot corresponds to one cycle of the clotting frequency, or the interval between the centers of two separate dots transmitted as close together as possible also corresponds to one cycle of the dotting frequency. It is assumed in the above discussion that the keying code used is the international Morse code or some equivalent code.

The keying device Ira is arranged to alternately ground one or the other of two different taps on the battery 2a, so that varying potentials are applied by this battery to the conductor 3a which is connected thereto. In the preferred embodiment of the invention the taps of the battery are so selected that the potential of the wire 3a varies between 45 volts negative and 3 volts negative according as the keying contact of the device la is in its lower or upper position as shown in the drawings. These varying potentials on the conductor 3a are used to control the bias of a tone keyer and amplifier 4a, which comprises an amplier, preferably of the pushpull type, s-o designed that under the normal bias conditions of -45 volts on the wire 3a the amplifier will be ineffective to transmit signals, Whereas under the less negative bias condition of -3 volts the amplifier will be operative. An audio tone generator 5a constantly supplies audio frequency carrier wave energy to the keying amplier 4a, so that the output of this amplifier delivers pulses of carrier keyed in accordance with the operation of the keying device la. The

key modulated audio carrier from the output of amplifier 4c, is applied through adjustable volume control potentiometer 61a; to the input of the shaping filter la. This shaping lter is of the band-pass type designed to pass the audio frequency tones of generator 5a and also to pass without any considerable attenuation those side bands of the key modulated audio carrier which correspond to signal components of the dotting frequency or lower. This filter la, however, is designed to substantially block the more remote side bands of the modulated audio frequency carrier which' correspond to the higher harmonics of the dotting frequency in the signals from keying device la. In the preferred embodiment of the invention, the lter 'la has a nominal passband of 300 cycles per second in width and has a suciently sharp cut-olf so that at a keying speed of 300 words per minute corresponding to a clotting frequency of 120 cycles per second, the filters discriminate against the side bands corresponding to the third harmonic of the dotting frequency by about 9 decibels as compared with the side bands corresponding to the dotting frequency itself` In other words, the attenuation for the side bands corresponding to the third harmonic of the clotting frequency is about 9 decibels greater than the corresponding attenuation for the side bands corresponding to the fundamental dotting frequency itself.

The exact transmission characteristics of the shaping lters 1a, lb, lc, etc. are dependent upon the particular values of audio frequency for the respective carriers. For an audio frequency carrier of 1000 cycles per second which may suitably y,be used for the tone signal sender A, the filter P. P. S. Hfd. Mfd. Mfd. A-lOOO '22a-.110 75m-.265 7311-.110 Bl700 72b-.03308 751;-.156 731)-.03308 C-2400 T20-.01566 75c-.1102 73o .01566 :l-3100 72d-.00912 'ld-.0854 7311-100912 E SSOO 720-.00596 756-.0696 73e-.00596 F-4500 'F-.00420 75f-.0588 'wf-.00420 (Tv-5200 T20-.00312 75g-.0510 7357-.00312 H-5900 72h-.00240 75k-.0448 73lL-.00240 A number of further tone signal senders C, D, E, F, G and H (not shown) are also provided. These other tone signal senders are similar to the senders A and B shown in the drawing, but the carrier frequencies of the respective tone generators and the condenser values of the respective filters are different, these values being chosen so that the lters will correspond with the carrier frequencies. The carrier frequencies should be separated by a number of cycles greater than twice the dotting frequency, preferably greater than four times the dotting frequency, but the separation should be sufliciently narrow so that all the channels may lie within the audio frequency spectrum. Suitable values for the carrier frequencies and filter condensers of the various tone signal senders A, B, C, D, E, F, G and H are given in the above table. Preferably the inductances of all the lters are alike in the interest of interchangeability and economy, each inductance being .318 henry as in lter la.

The output of the lters of the different ltone signal senders A, B, etc. are connected directly in parallel as shown in Fig. 1, and all these parallel connected outputs are coupled through transformer 8 to the ultra-high frequency transmitter 9 by way of the shielded high frequency cable I0. The ultra-high frequency transmitter 9 comprises means for generating an ultra-short Wave signal, and modulating this signal in accordance with the combined signal energy received from the cable I0. Thus an ultra-high frequency wave which is preferably of the order of megacycles and which is modulated in accordance with a mixed signal consisting of all the different key modulated carriers from the various tone signal senders A, B, C, D, etc. is delivered from the ultra-high frequency transmitter 9. This delivered wave is radiated over antenna Il which is preferably of a directional type and may, for example, be constituted by a V antenna or an array of V antennae.

Referring now to Fig. 2, the receiver illustrated in this figure comprises an antenna I2 for receiving the radiations from the antenna Il of the transmitter, and an ultra-high frequency receiver and detector I3 in which these radiations are received, amplified and detected in any well known manner to yield a signal corresponding to the mixture of key modulated carriers delivered to cable I0 in the transmitter of Fig. 1. This mixture of carriers is transmitted from the receiver and detector I3 over a shielded high frequency cable li to an audio amplifier l5 preferably through a variable attenuation pad I6 which comprises two ganged potentiometers whose windings are tapered so as to vary the attenuation of the signals entering the audio amplifier I5 while yet presenting a substantially constant terminating impedance to the high frequency cable III.

The output of audio amplier l5 of Fig. 2 is connected to a number of pre-filters Ita, lEib lh, which respectively constitute the input stages of the tone signal receivers A', B H Aas shown. In the preferred embodiment of our invention, the filter Ita comprises an inductance lSla, a condenser ISZa, a condenser ISSa and another inductance Ilia all connected in series, and a shunt condenser la across the line between condensers IEZCL and I63a. For simplicity and economy the inductances lla and ISIia are preferably similar to the inductances 'lla and 14a, of the filter 'la of Fig. 1, and the condensers l62a, l63a and Ia have the same values as the corresponding condensers Ha, 13a and 15a of the filter 'la of Fig. l. As will be more fully explained later on, a slightly different set of values is theoretically preferable for the filter la, but it has been found experimentally that values corresponding to the values used in filter *la are satisfactory for practical purposes when the carriers f are spaced 700 cycles apart and the interference noise conditions are not exceptionally severe. In most cases, therefore, it is preferred for the sake of economy, simplicit and standardization of parts to employ for the lter Ia components similar to the components of filter la. The other filters lGb lh are similarly designed so as to correspond to the shaping lters lby lh of Fig. 1, the inductances of all the filters lGa lh being therefore alike and the condensers being selected in accordance with the table of values for the filters 'la lh.

To the outputs of each of the filters la lh is connected an audio amplifier and limiter such as I'la, lb or lh, each such amplifier and i limiter preferably comprising a push-pull amplifier of which one push-pull stage has a bias derived from a grid leak such as lBa, lh lh, Icy-passed by a condenser such as lSa, lsb

. lSh. The characteristics of the audio amplier and limiter Ila, or of the other limiters l'lb I'lh, are such that under normal operating conditions the grid bias is somewhat beyond cut-off so that for signals below a certain given amplitude level no amplication occurs. Furthermore, for signals whose amplitude level is greater than the grid bias the amplifier-limiter l'la is designed to become substantially ineffective by virtue of the drawing of grid current since the impedance of the input to the grid is so high that upon the drawing of appreciable grid current the signals applied to the grid will be substantially prevented from further rising. Thus the characteristics of the audio amplifier I'Ia are such that for a given grid bias it will effectively transmit only those amplitude components which are above the critical level corresponding to cutoff for that grid bias and below the critical level corresponding to the point where thegrid grows positive with such bias. The characteristics of the amplifying tube of the limiter l'la are preferably such that the separation between the cutoff level and the level at which the grid grows positive remains substantially constant with varying grid bias. Thus the overall effect of the limiter l'la is to transmit only those amplitude components of the signals applied to it which lie between given upper and lower levels, and to automatically adjust the height of such levels, but not their separation, in response to variations in the mean value of the signals applied to the input of the limiter.

In order to further controlk such variations in the critical level of the different limiters l'la,4

l'lb I'lh, a common loop connection is provided for interconnecting the grid bias leads 2la, 2lb 2Ih of the different limiters through a corresponding number of loop resistors 20a, 20h 20h as shown. The effect of this loop connection through the resistors 20a, 201), etc. is to tend to equalize the grid biases of the various leads Ela, 2lb etc., and such equalization effect is most pronounced with respect to two adjacent channels, such as channel A and channel B, or channel C and channel D. If values of 100,000 ohms for the individual grid leads such as I8a, lh, etc. and 10,000 ohms for the loop resistors such as 23a, 20h 20h are used, the equalizing effect of the loop circuit will predominate over the individual bias influences of the separate resistors lSa, Ib, etc. so that the bias of one of the limiters which may not be receiving any signal at all will fall less than 25% below its normal ias if the other channels are still operating.

Such resistance values therefore provide a very great equalization of grid biases among the diferent amplier limiters. .A t the same time the resistors 20a, 20h 20h together with the condensers lila, ISI) lh form isolating filters to prevent a direct audio frequency interaction between the various limiters.

If it is desired to provide a still more complete equalization of the grid biases among the various amplier limiters, the resistors 20a, 20h 20h may be replaced by choke coils, but for practical purposes it has been found that the use of resistors is completely satisfactory.

The output of each limiter, such as lla, is connected to a final filter such as 22a, which is preferably designed to have a sharper characteristic than the corresponding pre-filter such as Ida. In order to distinguish between the characteristics of filters Ia and 22a as far as possible, itl

may be desirable in some cases to design these filters in accordance with different laws so that the nominal pass-band of the final filter 22a is appreciably sharper than that of the pre-filter Ita. For this purpose it may even be desirable to construct the pre-filter lia so as to have a wider pass-band than the shapinglter l'la. Under ordinary interference conditions, however, it has been found that satisfactory results can be obtained when all three filters la, l6a and 22a have the same nominal pass-band', but the nal lter 22a has a sharper cut-off. This result may be obtained by designing the final filter 22a, with a greater number of elemental sections than the prefilter Ia as shown' in Fig. 2. As shown in this figure, the final ilter 22a is equivalent to two filters such as Ic connected in series. Thus this final filter lav comprises a series inductance 22m corresponding to the inductance liia, a series condenser 222s corresponding lto the series condenser' S0211, a shunt condenser 22M corresponding to the shunt condenser |6511, a series inductance 221a which serves as the final inductance of the first section of this filter as well as the initial inductance of the second section of this filter, and therefore has an inductance twice as great as the inductance of |6|a or |64a, a condenser 22811 which serves as both the final condenser of the first section of the filter (corresponding to condenser I63a) and also as the initial condenser of the second section of the filter 22a (corresponding to condenser |62a), a shunt condenser 226a corresponding to condenser |65a, a series condenser 223a corresponding to condenser |63a, and a final inductance 224:1 corresponding to inductance |64a. It will be noted, therefore, that the components of filter 22a correspond to those of |6a with the exception of inductance 221er which is twice as great as either of the inductances of filter |6a and of condenser 223er which has half the capacity of the condenser |6211 of filter I6. Therefore it will be seen that the filter 22a corresponds to two filters such as IEa connected in series, the single inductance 221a and the single capacitance 228:1 being substituted in place of the condenser, inductance, further inductance and further condenser which would otherwise be serially connected together if two separate lters such as Ia were actually used.

In similar manner the filter 22h may correspond to two filters such as |617 connected in series, the filter 22h may correspond to two filters such as lh connected in series, and the remaining final filters (not shown) may likewise correspond to two of the corresponding pre-filters connected in series.

The output of the final filter 22a is coupled through a variable attenuation pad 23a to a full wave rectifier 24a which may also comprise audio amplification if desired. The variable pad 23a comprises two potentiometers connected as shown and ganged together so that the apparent impedance as viewed from the final filter remains substantially constant and equal to the filter impedance when the attenuation of the loss pad 23a is varied. The rectifier 24a is preferably of the push-pull type and has an adjustable bias so that it will respond only to signals above a given level, which level may be adjusted manually. Thus in connection with. the final filter 22a which rounds the dot and dash envelope of the corresponding carrier, the adjustable cut-off level of the rectifier 24a provides means for manually varying the weight or marking/spacing ratio of the rectified signals delivered from rectifier 24a.

The output of rectifier 24a is smoothed by smoothing network 25a consisting. of a choke and two condensers for eliminating the carrier frequency ripple which, in the case of full wave rectification of a 1000 cycle carrier would be a 2000 cycle ripple, and is then utilized for actuating a telegraph recorder or high power radio transmitter schematically represented by 26a.

Tone signal receivers B H' are essentially similar to the tone signal receiver A as indicated in Fig. 2, each of these tone signal receivers having a pre-filter and amplifier-limiter as previously described, and thereafter having a final filter, a variable loss pad, a full wave rectifier, a smoothing means and a radio transmitter or telegraph recorder as shown in the drawings. y

Fig. 3 represents the details of any one of the audio amplifiers and limiters such as |1a. |1b 11h. As shown in Fig. 3, this amplifier and limiter consists essentially of two push-pull amplification'stages of which the first stage has a self or cathode bias, the second stage has a bias produced by the flow of grid current through the appertaining grid leak resistor I8, this grid bias being, however, modified by the flow of current between the various limiters through the corresponding loop resistor 20 as shown in Fig. 2. Referring to Fig. 3, the tubes |13 of the first amplication stage may suitably be tubes of the so-called 89 type, in which three grids are provided, the two outermost grids being tied to the anode so that the tubes will act as triodes. The two tubes |15 of the final amplification stage which perform the limiting action, may suitably be tubes of the so-called 4l type, which are provided with screen grids in addition to the control grids thereof. The screen grids may be returned to the positive B supply as is shown. The interstage coupling transformer |14 is preferably a high quality audio transformer whose transformation ratio is such that the effective impedance of the secondary when the primary is connected to the plates of tubes |13 as shown, will be high in comparison with the impedance of the grids of the limiting tubes |15 when these grids become positive and begin to draw grid current, A transformer which has been found suitable for the interstage coupling transformer |14 is the type marketed under the trade name Kenyon and identified by the number Fl0145-1. The input transformer |12 preferably has an effective input impedance which is high in comparison with the input impedance of filter IGa, and a resistance 1| is shunted across the primary of this transformer to make the effective input impedance of the complete limiter-amplifier match the iterative impedance of the filter |6a while at the same time minimizing the variation of this effective input impedance. The output transformer |16 similarly has an effective output impedance higher than the iterative impedance of the final filter 22a and this output impedance is matched to the iterative impedance of the final filter by meansv of shunting resistor |18 and series resistor |11 which together render the effective output impedance substantially independent of variations in impedance of the tubes |15.

Referring now to Figs. 1 and 2, taken together to show the complete system, it will be seen that transformer 8, cable I0, transmitter 3, antenna Il, receiving antenna l2, receiver I3, cable |4, adjustable pad |6 and amplifier I5, together constitute a common transmission path over which all the telegraphic signals are transmitted from the keying devices la, Ib, etc. to the recorders or retransmitters 25a, 2Gb 26h. In its broadest aspects, therefore, each signal channel may be considered as comprising one tone signal sender such as A, the common transmission path above traced, and one tone signal receiver such as A'. Considered in slightly more detail, each channel may be considered as comprising a source of signals such as la, means for transferring said signals to an audio frequency carrier while eliminating the high harmonic components of said signals, these means including 5a 4a, 6a, and 1a, a common transmission path which includes 8, |0, 9, I2, I3, I4, I6 and |5, as above explained, a separating filter such as IEa, a self-adjusting limiter such as Ha, a nal sharper filter such as 22a, and means for further limiting, detecting and utilizing the waves comprising the amplifier-rectifier 24a with its ad- -justable threshold level, and the transmitter or recorder 26a.

It will be noted that in the above analysis the tone generator a, the keying amplifier 4a and the shaping filter la, have been considered together as a single modulating and restricting means for transferring the key signals to a suitable audio frequency carrier while eliminating the higher harmonic components of the key signals. Although in the preferred arrangement above described the tone generator is separate from the keying modulator and the shaping filter follows the modulation stage so as to act upon the carrier rather than upon the keying signals themselves, satisfactory results may also be obtained by combining the generator and modulator and/or by connecting the shaping filter so as to act upon the keying signals instead of on the modulated carrier. One advantage of positioning the shaping filter` subsequent to the modulator is that in this position the lter can be more cheaply constructed. The reason for this is that the function of the shaping filter is not merely to round the keying signals in an indiscriminate fashion, but rather to eliminate the harmonics of these signals which extend outside a certain reasonably definite frequency limit. Thus if an equivalent filter were to be connected so as to act on the key signals instead of on the modulated carrier, this filter should preferably be not of the brute force type but of the tuned type, and for such a tuned low pass filter designed to cut-o at a frequency of the order of 150 cycles, it is clear that the components would be far more costly and bulky than rThe principles of operation of the above described system may be briefly traced as follows: (1) Each of the tone signal senders produces an audio frequency carrier modulated with the essential components of a series of high speed telegraph signals, but restricted so as not to include side band frequencies corresppnding to the higher harmonic components of such telegraphic signals. Specifically, for a keying speed of 300 words per minute corresponding to a dotting frequency of 120 cycles per second, the fundamental dotting frequency ls attenuated by less than 1 decibel with respect to the carrier frequency itself, whereas the side band corresponding to the third harmonic of the dotting frequency is attenuated by almost decibels more than the carrier itself. It should be noted that this is in sharp distinction to the practise ordinarily accepted in the art, wherein it is ordinarily considered that a carrier channel must have sufficient width to pass at least the third and fifth harmonics of the dotting frequency in order to obtain reliable and accurate transmission of the signals.

The various modulated carriers from the tone signal senders A, B, etc. are transmitted over the common transmission path extending from transformer 8 to the output of audio amplifier l5 in a manner well known per se. It should be noted, however, that during transmission through this common transmission path the signals are subjected to a very considerable amount of fading, and in addition a Very considerable amount of interfering noise is also introduced. Furthermore, it should be noted that unless very expensive square band-pass filters are employed for the shaping filters la, 1b,'etc. the different modu- Cil lated carriers will Vto a slight extent overlap in spite of the fact that these carriers are spaced 700 cycles apart and the nominal pass-bands of the filters are only 300 cycles. Also unless the transmitter and receiver of the common transmission path are absolutely linear in their characteristics, a certain amount of cross-modulation between the signals of one modulated carrier and vanother modulated carrier will inevitably occur in the common transmission path.

The various component parts of the receiver of Fig. 2 must therefore perform the several functions of separating a number of modulated carriers which slightly overlap, and deriving from each of these carriers a clear undistorted signal free from fading and also free from cross-talk and extraneous interfering noise. Furthermore, the signals are, as previously mentioned, restricted so as to include only the most essential components of the original telegraphic key signals, and this fact further complicates the problem of receiving these signals in undistorted fashion and especially complicates the problem of compensating for fading without introducing a corresponding distortion of the weight or marking/spacing ratio of the signals. Also it should be noted that the separation of the various carriers is only slightly greater than the essential separation for transmitting the necessary signal side bands, since at a keying frequency of 300 words per minute the side bands which correspond to the dotting frequencies themselves occupy more than one-third the frequency band between two carriers. l

In order to meet the above outlined conditions and satisfactorily reproduce each of the separate series of telegraphic signals in clear and undistorted form free from noise and cross-talk and fading, the receiving system of Fig. 2 embodies a number of novel features, some of which have independent utility in themselves, but all of which act together and depend on one `another in somewhat complex fashion. For simplicity in understanding, in the following description of operation of the receiver, the various features thereof will be discussed separately so far as possible, but it should be understood that the novel features of the invention are not limited to the several features so separately discussed but include also a number of other features which are obtained by the complete combination and organization of parts shown in Figs. 1 and 2, but Whose theory of operation is not yet so fully understood as to be capable of analytical explanation.

One of the features of the present invention resides in the combination of the shaping filter la, the pre-filter 16a, and the limiter Ila, toi gether with the corresponding filters and limiters of rother channels such Ias B, C, D and the like. In accordance with this feature of the invention, the lter la is so correlated with the keying speed of the signaling device la, that only the essential components of the keying signals are transmitted as modulations of the carrier envelope, the second harmonic of the dotting frequency being very appreciably attenuated and the third harmonic ofthis dotting frequency being substantially suppressed before transmission to the common transmission path. The result of such restriction of the signal components is to very greatly round the signals, so that the dots and spaces between them together form very'nearly a pure sign wave, while the dashes have a substantially flat top over most of their length, but are rounded at the corners to about the same extent as the dots.

At the receiver the separating filter |6a may slightly increase the rounding effect of the signals, although if this separating filter |a is made somewhat broader than the shaping lter |1a, which is preferable for other purposes as later explained, such a further rounding action may be obviated. Finally in the limiter |1a the signals are so distorted that only those amplitude components lying between two given critical levels are transmitted, the rounded peaks and valleys which lie above and below these two levels being completely suppressed. By this combination of parts it is possible to transmit at very high keying speeds over a number of comparatively narrow multiplex carrier channels a number of separate series of square-topped keying signals, and to receive these keying signals in substantially square-topped form in spite of the fact that during transit lter means have been interposed for deliberately rounding these signals to an almost unrecognizable shape.

In accordance with a further feature of the invention,the limiter |1a of the above mentioned combination is arranged to automatically adjust the height of its two critical levels above mentioned, so that these levels constantly intersect the rounded signal waves at an amplitude roughly half-way between the peak amplitude of the incoming signal waves and the Zero amplitude thereof. By means of this feature not only are the waves transmitted from the limiter in clear square-shaped form in spite of the deliberate distortion of these signals to a sinusoidal form for the purposes of transmission, but further the weight or marking/spacing ratio of the telegraphic signals remains substantially undistorted in spite of the rounding and subsequent squaring of the waves, even though a very considerable amount of fading may occur in the common transmission path. This can be more clearly understood if it is recognized that fading merely varies the absolute amplitude of the waves transmitted through the common path, but does not essentially vary their shape. Therefore, since the limiter automatically adjusts its two critical levels so that these will lie roughly half-way between the peaks and valleys of the rounded signals, the variations of amplitude introduced by I fading will not appreciably vary the lengths of the dotsl or spaces between dots, and thus the Weight of the signals will not be altered by the combination of the shaping filter, the pre-filter, the fading action of the common path, and the squaring action of the limiter.

A further novel feature of the invention resides in the arrangement of the amplifier-limiter itself. Referring more particularly to Fig. 3, it will be seen that this limiter comprises two amplification stages of which the second one actually performs the limiting. The rst stage, which may in some cases be omittedvif the signal delivered to the limiter is of a suiciently high level, serves to amplify the signals so that the limiting action of the second stage will be effective. The second stage acts in two different manners to perform two different kinds of limiting, this stage serving on the one hand to reject signals below a minimum level corresponding to the signal voltage at which the grids of the tube become less negative than the cut-off value, and serving on the other hand to suppress amplitude components above the level corresponding to the signal voltage at which the grids of the tube |15 become positive and begin to draw grid current. It is clear that the values of these two levels both depend on the mean grid bias applied over grid return lead 2| to the two grids of the tube |15, the lower level of signal voltage corresponding to the voltage difference between the mean grid bias and the cut-off grid bias of the tube, and the upper level corresponding to the voltage diierence between the mean grid bias and the more positive value of grid bias at which appreciable grid current begins to be drawn.

Assuming for simplicity that al1 the limiters are working at about the same level so that the equalizing action of the loop through resistor 20 is ineffective, it is clear that the value of the mean grid bias on the grid return lead 2| depends essentially on the current through grid leak resistor I8. Primarily the mean value of current depends upon the percentage of the time that the grid is positive and upon the extent to which the grid goes positive during such time. In response to varied amplitudes of incoming signals produced by fading, therefore, the mean grid bias will tend to adjust itself in such a fashion as to maintain the critical upper and lower cut-oli levels at an intermediate value between the minimum and maximum signal levels, the upper cut-01T level at which the grid grows positive constantly intercepting the waves at a two-thirds point, or a three-fifths point, or at some other given point which divides the waves in a substantially fixed proportion, regardless of variation in the amplitude of the Waves. By suitably selecting the resistance value of grid leak I8 the upper critical level may be made to intersect the waves at any given fraction of its amplitude, and preferably the grid leak resistor is so selected that for signals of normal amplitude the upper critical level will intersect the waves as much above the center of the rounded signal envelope as the lower level is below the center of this rounded signal envelope. Thus when the amplitude of the waves varies through fading, the weight or marking/spacing ratio of the signals will remain substantially constant.

'Ihe amplitude of the waves transmitted through the limiter will likewise be independent of the amplitude of the incoming waves since the amplitude of the waves transmitted from the limiter will depend essentially upon the voltage difference between the cut-off bias of the limiting tubes (i. e. the voltage at which plate current be` gins to flow) and the grid current point of these tubes (i. e. the level at which appreciable grid current commences to flow).

The separation between-these two levels will depend primarily upon the plate voltage applied to the vacuum tube, and upon the type of tube employed. If the limiter tubes are of the soc'alled 41 type, it has been found that a plate voltage of approximately volts is suitable for the B supply |19. To a lesser extent also, the amplitude of the waves delivered by the limiter will depend upon the impedance of transformer |14. This impedance should preferably be such that with the primary connected to the plate of tube`|13 the effective impedance of the secondary becomes very large in comparison with the grid impedance of tube |15 as soon as the grids of these tubes b-ecome appreciably positive 'and start to draw any considerable amount of grid current.

It is thus clear that the total output of the limiter when properly adjusted will be invariable in amplitude and weight and that the` only variation caused by changes of amplitude in the incoming signals will be a slight variation in the slope of the sides of the square-topped and square-bottomed wave shapes delivered by the limiter.

The manner in which the various limiters are connected together through the loop circuit comprising resistors a, Zliby 20h inorder to partially equalize the grid biases of the Various limiters also constitutes a feature of the present invention. The provision of the equalizing loops maintains a substantial bias upon a limiter even when such limiter is not receiving any signals as when for example, when its corresponding channel is temporarily shut down. It should be noted that if no bias were maintained on such limiter during such an idle period, the critical cut-off level of that limiter would drop to Zero so that even the weakest signals would be passed by the limiter. Under such conditions extraneous noise and especially cross-talk noise resulting from overlapping of channels and from crossmodulation in the common transmission path, including transmitting, receiving and amplifying equipment, would pass throughthe limiter and might give rise to spurious signals in the telegraph recorder or radio transmitter 26 of the dead channel. The common equalizing loop, however, obviates the possibility of such extraneous noise by preventing the critical level of the limiter of a dead channel falling to Zero. Furthermore, the arrangement is such that if either of the two adjacent channels are operating the bias of the dead channel will be maintained at a reasonably high value, which is desirable because under such conditions the crosstalk power reaching the dead channel is higher than if the adjacent channels were also dead.

Another feature of the present invention is the combination of a pre-filter such as lila, followed by a limiter such as lla, which is in turn followed by another filter such as 22a, the nal filter being sharper either in respect to its pass-band or in respect to the completeness of its cut-off, than the corresponding pre-filter. It has been found experimentally that the reduction of static noise, especially the untuned type of static noise arising from sharp transient surges is considerably enhanced when the pre-filter is broader than the nal filter in such a combination. In fact, under conditions where only one channel was operating, so that the problem of cross-talk noise could be neglected and only noises of external origin were present, it was found that the complete elimination of the pre-filter 16a greatly reduced the total amount of noise delivered from the output of the nal filter 22a. This phenomenon appears inexplicable at first, but upon careful analysis proves to be perfectly logical.

It should be recognized that the energy of the ordinary static surge contains components of almost all frequencies extending over a very wide band, but ordinarily so related in phase that the result of all those frequencies is a single sharp peak or a few sharp peaks. If it be assumed that the minimum signal channel width which must be passed in order to intelligibly receive the desired signals is 300 cycles in width, there will be a certain fraction of the interfering noise which will fall within this band and which therefore cannot be eliminated by filters no matter how perfectly these may be designed. If the prefllter were designed as sharply and narrowly as possible, so as to eliminate all components of the interfering-signals excepting those which actually fell Within the required 300 cycle band, it

is clear that thereafter no further filtering could reduce the noise to signal ratio. Furthermore, the action of the limiter subsequent to such complete fltering would also be of no use unless the amplitude of the noise after such filtering was appreciably lower than the amplitude of the desired signals. If, however, the resulting noise after ltering were as great as the amplitude of the signals or greater than this amplitude, the limiter could at most reduce the noise and the signals to equal amplitude.

On the other hand, if the pre-filter is made very broad so that it passes noise components of all frequencies within a somewhat Wider band, say 800 cycles, for example, and if the noise, which is again assumed to be stronger than the signal, is passed through a limiter so that both the noise and the signals are delivered from the limiter at the same amplitude, then by the use of a further sharp lter restricted to a 300 cycle pass-band, it is possible to relatively reduce thenoise level as compared with the signal level by eliminating those components of the noise which lie outside of the essential 300 cycle signal band.

If it be assumed that the amount of noise energy which passes through an 800 cycle bandpass lter produces a transient pea-k which is 8/ 3 as high as the peak produced by the amount of energy which would pass through a 300 cycle pass-band filter, and if it be assumed that in neither case is the transient noise peak eliminated by the lower cut-off level of the limiter, it then follows that the improvement in signal to noise ratio which is effected by the limiter will be 8/3 as great in the case of the broad pre-filter as in the case of the narrow pre-lter. It is true that in both cases the apparent signal to noise ratio at the output of the limiter itself will be unity, but in the case of the broader pre-filter it is then possible by the use of a subsequent narrow filter having, for example, a 300 cycle pass-band, to further reduce the noise energy in the ratio of 8 to 3, whereas in the case where the pre-filter was as narrow as possible the unity signal to noise ratio delivered by the limiter itself cannot be further reduced.

Considered purely from the standpoint of static surge noise elimination, therefore, it would be desirable to make the pre-lter lea as broad as possible, and preferably to eliminate kthis filter entirely. In practice, however, in a multichannel system of the type herein considered, such an arrangement is not satisfactory because if the pre-lter lila is broadened so far as to include a very great amount of the adjacent channel signals the effect of the limiter will be to introduce a cross-modulation which cannot thereafter be eliminated. In practice, therefore, the pre-filter must be made suiciently narrow so that it does not pass too much energy of the adjacent channels. The limitation in this regard is not absolutely definite, but preferably the pre-filter should be sufficiently narrow so that the adjacent channel energy which is passed by it will not exceed about one-tenth or one-twentieth of the signal energy of the channel itself. It should be noted that if the available channel separationv is large in comparison with the essential width of the signal band which must be transmitted on each channel, the effectiveness of the static elimination feature of the present invention may be considerably increased. For example, if the channel spacing were 2000 cyclesandthe essential width vof the signal band of each channel were 300 cycles, the pre-filter could be designed to pass a band 3700 cycles in width while the final filter could be designed to pass only 300 cycles. In such a case the restricting filter in the transmitter should also be designed to substantially eliminate all components lying outside of the essential 300 cycle band.

Even with the carrier separation of 700 cycles chosen in the previously described embodiment of the invention, and with a keying speed of 300 words per minute which requires an. essential signaling band of about 300 cycles in width, the effectiveness of the static surge elimination feature could be somewhat increased if this were necessary, by designing the restricting or shaping filter in the transmitter so as to pass only the 300 cycle band required and so as to very sharply cut-off all frequencies lying outside such band. Then if the pre-filter 16a were likewise designed with a very sharp and definite cut-off, this filter could be arranged to pass a band 1100 cycles in width while the final filter 22a could be limited to a 300 cycle band with a sharp cutoff outside of this band.

In practice, however, it is not ordinarily necessary to go so far, and the additional cost of designing the transmitter shaping filter, the prefilter and the final filter so as to have very sharp square cut-off characteristics is ordinarily not justified unless the conditions of static noise and the limitations as to channel separation make it essential. In the preferred embodiment earlier described, therefore, the transmitter shaping filter, the pre-filter and the final filter are of the simple T type, the transmitter lter'and the preiilter consisting merely of one section and the final filter consisting of two sections. It has been found experimentally that even this arrangement wherein the filters have the same nominal pass-band, but wherein the pre-filter has a smaller number of sections than the final filter so that the band of frequencies actually passed by the pre-filter is somewhat broader than the band passed by the final filter, is capable of producing the advantageous effect above discussed. At the same time, the preferred design above disclosed has the advantages of standardization of parts in all filters and of great simplicity and low cost in the filters.

It will be understood, however, that in respect to this feature the invention is not limited to an arrangement wherein the filters have the same -nominal pass-band characteristics and wherein the difference in pass-band width is attained by varying the number of sections since as previously explained, the principle can be still,

more effectively carried out by the use of more expensive filters having different nominal passband characteristics.

It should further be noted that although it is preferred to employ harmonic restricting means at the transmitter, in connection with the ccmbination of a broad pre-filter, a narrow final lter and a limiter therebetween for eliminating sharp static transient surges, nevertheless in certain cases where the carrier separation is so great that the pre-filter can be made adequately broad without the necessity of restricting the signal components of the adjacent channels of the transmitter, this static surge elimination feature may be practised without the use of transmitter filters. In most cases, however, it will be found that the use of transmitter filters is desirable in cut-off level of the limiter.

order to permit a further broadening of the prefilter in the receiver.

A further feature of the invention is the combination of a pre-filter such as |60, with a limiter such as I'la, which is adapted to cut-off amplitude components lying below a certain critical level. It will be noted that in the above discussion of the sharp static surge elimination .feature, it was the upper cut-off level of the limiter which was primarily considered, and that the interfering noise with respect to which this feature was most advantageous was of the sharp peak type having an amplitude very high in comparison with the signal amplitude. In practice, however, it has been found that there is also a great deal of noise which may he called background noise, resulting both from cross-talk and from more or less continuous interfering noise sources in the neighborhood, which is more or less of the same order of magnitude as the signal amplitude and which has such a frequency distribution that after filtering through a prefilter such as lc the voltage amplitude of this noise is considerably lower than the voltage amplitude of the desired signals. In respect to noise of this type, the combination of the prefilter |6a with the lower level cut-01T characteristic of the limiter Ha, provides an unusually effective means for eliminating interference.

Even if the background noise has an amplitude considerably greater than the signal before filtering, it is ordinarily so distributed throughout the frequency spectrum that the components thereof which lie within the pass-band of the pre-filter are somewhat weaker than the signals. Nevertheless, the mere reduction of amplitude of this noise by filtering in the pre-filter is not sufficient to provide a' satisfactory noise-free signal. By the combined action of the pre-filter and the lower cut-olf characteristic of the limiter, however, this noise may in.most cases be completely eliminated, since its amplitude is first reduced by the filtering action of the filter and then is in most cases insuihcient to pass above the lower Thus by the use of a moderately broad filter together with a limiter which rejects signals below a certain level, interfering noise having an amplitude considerably greater than the signal amplitude itself may be not only reduced but actually completely eliminated.

It should also be noted that this arrangement is still more effective when the limiter is arranged to have an automatically adjustable level as previously described, so that its lower cut-off level tends to adjust itself automatically with respect to the amplitude of the incoming signals. The use of such a self-adjusting lower cut-off limiter with a pre-filter such as Ia has the advantage that during periods of weak signal reception the lower cut-off level will not be so high as to eliminate the signals, while at the same time during the ordinary period of strong signal reception, the lower cut-off level will be automatically raised so as to increase the effectiveness of the background noise elimination feature.

It should be noted that the background noise elimination feature, whereby interferences having amplitudes only a few times greater than the signal amplitude are eliminated by filtering and thus reducing their amplitude below the lower cut-01T limit of the limiter, operates in an entirely different manner from the surge noise elimination feature for eliminating noises of very brief duration which have amplitudes tremendously greater than the desired signal amplitude. In some ways the requirements forthe most effective action of these two features are contradictory since the surge elimination feature is most effective when the pre-filter is broad whereas the background elimination feature is most effective when this pre-filter is narrow. In practice, however, these two features can be combined very satisfactorily so that together they coooperate to eliminate almost all noise of the background type and to so reduce the rare surges of the sharp transient type, that these do not seriously interfere with signaling. In order .to render both features simultaneously most efiicient, it is desirable to bring the upper and lower critical levels of the limiter asY close together as practical, and this may be done by using a low plate voltage on the limiter tube so that the cut-olf grid bias does not differ very greatly from the more positive voltage at which grid currents begin. The breadth of the pass-band of the prefilter may then be selected to be sufficiently wider than the effective pass-band of the final lter so that the extra frequencies of a given static surge which are passed by the pre-lter but not by the final filter represent such a substantial proportion of the energy of the surge, that upon removal of these extra frequencies in the final lter the noise amplitude will become substantially lower than the signal level. Thus for all signals whose amplitude is so high that after the preltering stage they still exceed the upper limit of the limiter, the additional filtering reduction of the final lter 22a will reduce these signals to a tolerable amplitude. At the same time, the prefilter can readily be made so sharp that the number of signals which after passing through it are still higher than the lower level of the limiter is comparatively small so that the majority of the noise will be completely eliminated by the background elimination feature, and only occasional brief surges will have to be taken care of by the surge elimination feature.

In some cases where the maximum interference amplitude is sufficiently low so that the background noise elimination feature is sufficient to remove all objectionable interference, the nal lter 22a may be connected ahead of the amplifier-limiter lla, thus increasing the effectiveness of the background noise elimination feature and the cross-talk discrimination. In installations where such a modification is feasible by reason of the absence of high amplitude transient surges of static, the further advantage is gained that the signals delivered from the limiter are substantially square-topped, having practically the same shape and proportion as the signals originally produced by the keying device la. Thus if these signals are rectified they may be applied' to an ordinary telegraph recorder even if this recorder is not designed for receiving round-topped waves. Thus in installations where the common transmission path is constituted by a physical line or by a radio channel reasonably free from high amplitude static interference, this modified system provides a particularly advantageous multi-channel set of connection paths between a number of high speed telegraph key senders such as la, lb, etc. and a number of recorders such as 26a, 2Gb 26h. In such a system the carrier channels may be spaced still more closely since the pre-lter need not be broader than the essential pass-band necessary for passing the side bands corresponding to the dotting frequency 1tself,or possibly to a frequency fifty per cent higher than the Vdotting frequency. The harmonies of the dotting frequency, however, may be completely eliminated by the shaping filters of the transmitter, so that the envelopes of the modulated carriers` actually transmitted over the common transmission path lose their flat-topped form and in the case of a series of successive dots become substantially sine waves. Then by virtue of the reshaping characteristic of the limiter which restores both the flat tops and the 'flat valleys of the waves, the harmonics so eliminated in the transmitter are recreated in the .receiver so that an ordinary recorder adapted for operating with square-shaped waves can reliably record the received signals. Y

By virtue of this feature of the invention, the number of channels which can be transmitted by the modulated carrier principle within 'a xed frequency spectrum of given width is considerably increased. This feature is of particular importance in the case of wire transmission where the total frequency spectrum available is restricted to 4000 or 7000 cycles or so, and where it is desired to transmit simultaneously a large number of Vhigh speed telegraph signals over onev single channel. In accordance with the prior art practice, such multiplex transmissions by the modulated carrier principle have always required a carrier separation sufiicient to transmit not only the dotting frequency but also thev third and fifth harmonics thereof. Thus in accordance with this feature of the present invention, the total number of channels which can be transmitted over a given frequency spectrum by the modulated carrier principle is more than doubled.

Although in the above description it has been assumed that the carrier frequencies should all lie within the audio frequency spectrum, the invention can be practised'with carriers of superaudible or radio frequencies.

Although we haverdescribed certain embodiments of our invention for the purpose of illustration, it will be understood that adaptations' alterations and modifications thereof occurring to one skilled in the art may be made without departing from the scope of the invention as defined in the appended claims.

What we claim is:

1. kA system for transmitting a series of tele- `graphic signals having a given maximum dotting frequency, comprising a transmission path, con verting means controlled by said signals `for delivering to said path a carrier modulated with said signals, saidconverting means including restricting means for restricting the sidebands substantially to those frequencies corresponding to signal components below the third harmonic of the dotting frequency, means for deriving said restricted modulated carrier from said path at a remote point, and means for reconverting said derived modulated carrier into a series of squareshaped telegraphic signals, said reconverting means including limiting means lfor recreating the higher harmonic components eliminated.

2. A system for transmitting a number of telegraphic signals having a given maximumdotting frequency, comprising a common transmission path subject to variations in transmission attenuation, means for delivering to said pathV a number of adjacent carriers modulated with said signals but having sidebands restricted substantially to those frequencies corresponding to signal components below the third harmonic of the dotting frequency, means for receiving from said path and separating said modulated carriers,

means for suppressing all amplitude components of each such separated modulated carrier which lie outside a given amplitude range, and means for detecting the unsuppressed components of each such separated modulated carrier whereby telegraphic signal waves of a constant order of amplitude and containing substantial components of the third and fifth harmonics of said dotting frequency are produced. l

3. A multi-channel telegraph system comprising a plurality of keying means for simultaneously producing a plurality of seri-es of sharply defined keying signals each having a given dotting frequency, a converting circuit connected to each one of said keying means, said circuits delivering a corresponding plurality of modulated carriers of dilerent frequencies each modulated with keying signals, each of said converting circuits including shaping means for restricting the sidebands of said delivered modulated carriersV substantially to those frequencies corresponding to signal components below the third harmonic of said dotting frequencies, a transmission channelV for transmitting all said restricted modulated carriers to a remote station, a plurality of filters at said remoite'station for separating said modulatedcarriers from each other, and a plurality of reconverting circuits each connected to one of said filters for reconverting each of said restricted modulated carriers into a series of substantially sharply dened signals similar to the signals produced by the corresponding keying means, each of said reconverting circuits including signal limiting means for recreating the higher harmonic components eliminated by said shaping means. I

4. A system according to claim 3, wherein each of said converting circuits which includes shaping means comprises oscillator means for producing a corresponding .carrier frequency, modulator means for modulating said carrier with the corresponding keying signals, and a bandpass shaping filter connected to said modulator means to pass only the modulation frequencies corresponding to signal components below the third harmonic of the corresponding dotting frequency.

5. A system for transmitting a number of telegraphic signals having a given maximum dotting frequency, comprising a common transmission path subject to variations in transmission attenuation,A means for delivering to said path a number of adjacent carriers modulated with said signals but having sidebands restricted substantially to those frequencies corresponding to signal components below the third harmonic of the dotting frequency, means for receiving from said path and separating said modulated carriers, means for suppressing all amplitude components of each such separated modulated carrier which lie above or below given amplitude limits, and means for detecting the unsuppressed components of each such separated modulated carrier whereby telegraphic signal waves of substantially constant amplitude and of square-shape containing substantial components of the third and higher harmonics of said' dotting frequency are produced.

6. A multi-channel telegraph systemy comprising keying means for simultaneously producing a plurality of series of substantially square-topped keying signals each having a given dottingv frequency, a separate converting circuit connected to each one of said keying means for delivering a corresponding plurality of modulated carriers of different frequencieseach modulated with thecorrespondingseries ofkeying signals, each ofl said converting circuits-including shaping means for restricting the sidebands of the delivered modulated carrier substantially -to thosey frequen cies'corresponding to signal components below the third harmonic of the dotting frequency, a

transmissionv channel for transmitting all saidfre` stricted modulated carriers to a remo'testation, a plurality of ltersat said remote stationA for separating said plurality of modulated carriers' from each other, ar corresponding plurality of limiting means for passing only those amplitude components of the corresponding separated modulated carrierswhich lie above given lower amplitude limits, and means for detectingsaid trans--v mitted amplitude components to yield signals4 substantially free from interferencev and` having. substantially flat-bottomedform.

7. A system for transmitting a number of telegraphic sig-nalsfhaving a given maximum dotting frequency, comprising a common transmission path subiect to variations in. transmission attenuation, means for delivering to said path a number of adjacent carriersmodulated withsaid signals but having sidebands restricted substantially to those frequencies corresponding to signal components below the third harmonic ofthe dotting frequency, meanslfor receivingthe carriers from said path, a` rst band-pass lter for each carrier coupled to saidV receiving means for separating such received modulated carrier fromthe others, amplitude limiting means for each carrier following the corresponding first filter for so distorting such modulated and separated carrier as to'lirnit the amplitude components thereof Ito those which lie between given amplitude levels, means for varying said given levels of each limit-v ing means responsive to variations in mean amplitude of the corresponding carrier, and means for controlling such variations of said levels of each limiting means in dependence on the mean amplitudes of the other carriers whereby a number of carriers modulated with substantially square-topped signals ofY substantially constant amplitude are derived from said path.

8. Ai system for transmitting a number oftelegraphicsignals having a given maximum dotting frequency, comprising a commonv transmission path subject to variations in transmission .attenuation, means `for delivering to said path a number ofadjacent-carriers modulated with said 'signals but having sidebands .restricted substantially tothose frequenciescorrespondingto. signal components below the third harmonic of theV dotting frequency, means vfor receiving from said path and separating said modulated carriers, limiting means for transmitting only those amplitude components of each such separated carrier which lie on one side ofy a given amplitude limit, means for varying the level of each such limit inl response to variations in the mean am? plitudev of incoming signals, and means for detecting the transmitted amplitude components of each such limited modulated carrier.

9, A receiver for deriving a number of series of telegraphic signals from a corresponding number of adjacent carriers each modulated with one of said series of signals, comprising common receiving means for all said carriers, a number of iirst band-pass filters coupled thereto for separating said carriers, a number of amplitude limiting devices following said first lters for limiting the amplitude components of each carrier to those which lie on one side of a given amplitude level, means for varying said given level of each limiting device responsive to variations in mean amplitude of the corresponding carrier, means for controlling such variations of said level of each limiting device in dependence on the mean amplitudes of the other carriers.

10. A multiplex radio system for transmitting and receiving a number of telegraphic signals having a maximum dotting frequency, comprising means for radiating a number of adjacent carriers modulated with said signals but having side bands restricted substantially to those frequencies corresponding to signal components below the third harmonic of the dotting frequency,

PAUL F. BYRNE. ARMIG G. KANDOIAN. 

